CN1077884C - Process for prepn. of urea - Google Patents

Abstract

In the present process for the preparation of urea, an off-gas stream released during the synthesis of melamine in a high-pressure melamine process which consists predominantly of ammonia and carbon dioxide, is introduced into at least one high-pressure section of a urea stripping plant and is used in the synthesis of urea. The off-gas stream can be used directly without any further treatment.

Description

Urea preparation method

Background of the Invention

1. Field of invention

The present invention relates to a process for the production of urea in which the gaseous stream released from the melamine production process, consisting essentially of ammonia and carbon dioxide, is recovered and used without further treatment in the urea synthesis of the urea process. More specifically, the recycled gas stream is fed to the high pressure section of a urea plant.

2. Related technical description

Urea can be produced by feeding ammonia and carbon dioxide into a synthesis zone at a suitable pressure, eg, 12.5-35 MPa, and a suitable temperature, eg, 160-250°C. Ammonium carbamate is first formed according to the following reaction:

Ammonium carbamate is then dehydrated to urea according to the following equilibrium reaction:

The extent to which the latter reaction proceeds depends primarily on the temperature employed and the excess ammonia. The solution obtained as reaction product consists of urea, water, ammonium carbamate and free ammonia. Ammonium carbamate and ammonia need to be removed from the solution. Once removed, they are usually returned to the synthesis area. The synthesis zone may comprise separate ammonium carbamate and urea production zones. However, the two zones can also be combined in one device.

Urea can be produced in a conventional urea plant. In conventional high-pressure urea plants, the decomposition of ammonium carbamate not converted to urea and the removal of the usual excess ammonia is carried out at pressures between 1.5 and 10 MPa, which are substantially lower than in the urea synthesis reactor. The synthesis reactor typically operates at a temperature of about 180°C to about 210°C and a pressure of about 18 MPa to about 30 MPa. Ammonia and carbon dioxide are fed directly to the urea reactor. In conventional high pressure urea processes, the _NH3 / _CO2 molar ratio (N/C) in urea synthesis is generally between about 3 and about 5. Unconverted reactants are recycled to the urea synthesis reactor after expansion, separation and condensation.

A variant of the traditional process for preparing urea is described in GB-A-1309275. In the process, the off-gases obtained during the production of melamine in the high-pressure melamine process, often also called off-gases, are used for the synthesis of urea. Melamine tail gas is mainly composed of ammonia and carbon dioxide. The tail gas stream from the gas/liquid separator of the melamine plant is only sent to the low-pressure section after passing through the scrubber, that is, the first low-pressure urea synthesis section. In this low-pressure section, ammonia and carbon dioxide from the melamine plant are used to prepare a urea solution in an additional reactor. This urea solution is then compressed and sent to the high pressure section of the same urea plant.

The process of GB-A-1309275 has many disadvantages. An additional reactor is required because the tail gas stream supplied by the melamine plant, even if it comes from a high-pressure melamine process, is too low in pressure to be used directly in a conventional high-pressure urea plant. Furthermore, one or more additional pumps are required in order to transfer the urea produced in the first low-pressure urea synthesis section to the high-pressure urea synthesis section.

Despite various efforts to efficiently integrate urea and melamine production plants, there remains a need for an industrially simple and cost-effective method for recovering tail or waste gases from high-pressure melamine plants, consisting mainly of ammonia and carbon dioxide, and Directly used in high-pressure urea plant.

Summary and purpose of the invention

The present invention provides an attractive solution to the various requirements imposed by the industry by efficiently utilizing the tail gas stream from the high pressure melamine process directly in the high pressure section of the urea extraction plant.

The tail gas stream from the high pressure melamine process consists mainly of ammonia and carbon dioxide. "Mainly" means that more than 90% by weight of the tail gas stream consists of ammonia and carbon dioxide, preferably more than 95% by weight. The off-gas stream may also contain small amounts of eg melamine, urea, isocyanic acid and/or hydrogen. The _NH3 / _CO2 molar ratio in the tail gas stream is about 2 or higher, preferably between about 2.2 and about 4.

The high-pressure section of the urea extraction plant can be, for example, a urea reactor, an extractor, a carbamate condenser, an additional pre-extractor placed between the urea reactor and the extractor, a and a flash evaporator in line between the carbamate condenser or any such equipment.

One object of the invention relates to improving the efficiency of a high pressure urea plant. This object can be achieved by using the off-gas stream obtained from the high-pressure melamine plant in the high-pressure section of the urea extraction plant, this off-gas stream mainly consisting of ammonia and carbon dioxide, practically free of water. The result of this is an increase in efficiency compared to the supply of an aqueous carbamate stream from a melamine plant to a urea plant.

Yet another related objective is that the off-gas stream from the melamine plant does not require an absorption or concentration step before entering the urea plant. This is possible in the present invention because the off-gas gas stream is already virtually free of water and has a sufficiently high pressure.

Yet another object is to obtain higher energy efficiency in urea production. This is possible with the present invention because the extra heat released during the condensation of the off-gas stream from the high pressure melamine plant can be recovered and used to produce additional (low pressure) steam.

A brief description of the drawings

Fig. 1 is the flow chart of synthetic urea and melamine according to the present invention, it is from the tail gas of high-pressure melamine plant to the carbamate condenser of urea plant by high-pressure melamine plant circulation;

Fig. 2 is the flow chart of synthetic urea and melamine according to the present invention, it is from the tail gas of high-pressure melamine plant to the flash evaporator that urea plant is installed between extractor and carbamate condenser by high-pressure melamine plant circulation;

Fig. 3 is the flow chart of synthetic urea and melamine according to the present invention, and it is recycled to the extractor of urea plant by the tail gas of high-pressure melamine plant by high-pressure melamine plant;

Fig. 4 is the flow chart of synthetic urea and melamine according to the present invention, it is from the tail gas of high-pressure melamine plant to be installed in the pre-extractor between urea reactor and extractor by high-pressure melamine plant circulation to urea plant;

Fig. 5 is the flow chart of synthetic urea and melamine according to the present invention, it is directly recycled to the high-pressure pipeline of urea plant by the tail gas of high-pressure melamine plant by high-pressure melamine plant;

Figure 6 shows in more detail the tail gas stream feed to the high pressure pre-extractor in a urea plant according to the invention.

Detailed Description of the Preferred Exemplary Embodiments of the Present Invention

The invention relates to the production of urea in a urea extraction plant having at least one high-pressure section, wherein at least one high-pressure section of the urea extraction plant is supplied with an off-gas stream released during the high-pressure synthesis of melamine, wherein the off-gas stream consists essentially of ammonia and carbon dioxide.

In the simplest and preferred embodiment of the invention, the tail gas stream is fed to the carbamate condenser in the high pressure section of the urea extraction plant or to the line between the extractor and the carbamate condenser.

The pressure of the tail gas stream supplied by the high pressure melamine plant is generally higher than about 12.5 MPa. Generally, the pressure is less than about 80 MPa, preferably less than about 40 MPa, more preferably less than about 20 MPa. Specifically, the pressure of the tail gas stream from the high-pressure melamine plant is about 0 to about 10 MPa, especially about 0-3 MPa, more especially about 0-2 MPa higher than the pressure in the urea reactor. The temperature of the off-gas stream is generally above 160°C, preferably above 175°C. The temperature of the tail gas stream is generally below 285°C, preferably below 275°C, more preferably below 235°C.

As indicated here, a urea extraction unit generally refers to a urea plant in which the decomposition of ammonium carbamate not converted to urea and the removal of carbon dioxide and usually excess ammonia are carried out at substantially the same pressure as in the synthesis reactor. This decomposition/removal process is carried out in an extractor with or without extraction medium. In an extraction process, carbon dioxide, ammonia, or both can be used as extraction gas before being fed to the synthesis reactor. The extraction process takes place in an extractor, which can be installed downstream of the reactor. The solution produced in the urea reactor contains urea, ammonium carbamate, water, ammonia and carbon dioxide. The solution can be extracted with additional heat. The solution can also be extracted using thermal extraction techniques, in which the decomposition of the ammonium carbamate and the removal of the ammonia and carbon dioxide present from the urea solution take place solely by adding heat. The ammonia and carbon dioxide containing stream from the extractor is returned to the reactor through the carbamate condenser. Reactor, extractor and carbamate condenser are relatively important components in the high-pressure section of urea synthesis.

In the urea extraction plant, the synthesis reactor is preferably operated at a temperature of about 160 to about 220°C and a pressure of about 12.5 to about 17.5 MPa. Typical N/C ratios during extraction device synthesis are between about 2.5 and about 4.

The present invention is therefore applicable to the widely used process for the preparation of urea by the urea extraction process described, for example, in European Chemical News, Urea Supplement, of 17 January, pages 17-20, the complete disclosure of which is hereby incorporated by reference . In this process, a urea synthesis solution is produced in a synthesis zone at high temperature and pressure and, when heat is added, is subjected to extraction treatment at synthesis pressure by countercurrent contact with gaseous carbon dioxide. During this extraction operation, most of the ammonium carbamate present in solution is decomposed into ammonia and carbon dioxide. These decomposition products then escape from the solution in gaseous state and are removed together with small amounts of water vapor and carbon dioxide for extraction. This extraction process can be carried out using carbon dioxide (gas) as described, for example, in US 3,356,723, the entire disclosure of which is hereby incorporated by reference. Extraction can also be performed using thermal extraction techniques, or with ammonia in the gaseous phase as the extraction gas. Additionally, extraction can be performed using a combination of the extraction techniques described above. More than 95% of the gas mixture resulting from the extraction process is condensed and adsorbed in the carbamate condenser. The resulting ammonium carbamate is sent to the synthesis area for the production of urea. The gas mixture that is not condensed and absorbed may contain, for example, inert gases. Urea synthesis can be carried out in one or two reactors. For example, purified ammonia and carbon dioxide can be used in the first reactor. Pure ammonia and carbon dioxide plus recycled ammonia and carbon dioxide, or just recycled ammonia and carbon dioxide can be used in the second reactor. Synthesis is best performed in one reactor. The extraction of the urea synthesis solution with the aid of a gaseous extraction medium can also be performed in more than one extractor.

The carbamate condenser may be, for example, a so-called submerged condenser as described in NL-A-8400839, the entire disclosure of which is hereby incorporated by reference. In this case, the gas mixture to be condensed is fed into the shell-side space of the shell-and-tube heat exchanger, to which a dilute carbamate solution is also added. The released heat of solution and condensation is dissipated with the aid of a heat-absorbing fluid medium flowing through the tubes. For example, a suitable fluid medium is water, in which case the water can be converted to low pressure steam for use elsewhere in the method or apparatus. Immersion condensers can be installed vertically or horizontally. However, condensation is particularly advantageous in horizontally positioned submerged condensers, since the residence time of the liquid in the condenser is generally longer compared to vertically positioned submerged condensers. This leads to the formation of urea which raises the boiling point and, as a result, the temperature difference between the urea-containing carbamate solution and the cooling medium increases. Therefore, the heat transfer effect is better. A so-called pool condenser is an exemplary submerged condenser, one of which is described, for example, in Nitrogen No. 222, July-August 1996, pp. 29-31, the entire disclosure of which is incorporated by reference .

The condenser and synthesis zone can, if desired, be combined in one apparatus, as described for example in NL-A-100416, the entire disclosure of which is incorporated by reference. In the latter case, the production of ammonium carbamate and urea from carbon dioxide and ammonia can be carried out in a urea reactor at a pressure of about 12.5 to about 35 MPa. The urea reactor may have a horizontally placed condensation zone and a heat exchanger. An example of such a reactor is a so-called pool reactor described, for example, in Nitrogen No. 222, July-August 1996, pp. 29-31. Ammonia and carbon dioxide are fed to the urea reactor, most of which are condensed and absorbed in the urea synthesis solution. A heat exchanger is used to recover most of the heat released by exothermic condensation and generate water vapor. The residence time of the urea synthesis solution in the urea reactor should be selected so that the amount of urea obtained is at least 85% of the theoretically possible amount. Typically, this urea synthesis solution is then processed into urea solution or solid urea.

After the extraction operation, the extracted urea synthesis solution is expanded in several steps to low pressure and concentrated by evaporation. The molten urea thus obtained can be sent completely or partly to an "integrated" melamine plant for melamine synthesis. This urea and melamine operation is characterized as a joint operation.

Meessen et al., Ullmann's Encyclopedia of Industrial Chemistry, Volume A27, pages 333-365 (1996), including references cited therein, provide a general description of urea, urea plants and methods, the complete disclosure of which is hereby incorporated Reference.

Urea is the preferred starting material for the preparation of melamine. Urea is preferably used in a molten state. Ammonia and carbon dioxide are by-products generated during the preparation of melamine, which is prepared according to the following reaction equation:

        

The preparation of melamine can be carried out in the absence of catalyst at a pressure higher than 12.5MPa, generally lower than 80MPa, preferably lower than 40MPa, more preferably lower than 20MPa. The reaction temperature may generally vary between about 300°C and about 500°C, but preferably between about 350°C and about 425°C.

A melamine production plant suitable for carrying out the present invention may comprise, for example, a urea scrubber, a combined or uncombined reactor integrally or separately with the gas-liquid separator, optionally a reactor downstream of the reactor post-reactor storage tank or aging storage tank and a product cooling/product processing section. Crews et al., Melamines and Guanamines, Ullmann's Encyclopedia of Industrial Chemistry, Volume A16, pages 171-185 (1990), including references cited therein, provide a general description of melamine, melamine synthesis, and melamine apparatus, which are presented here The complete disclosure of this document is incorporated by reference.

In one embodiment of the invention, melamine is produced from urea in a plant comprising, for example, a urea scrubber K, a reactor L for producing melamine, a gas/liquid separator M and a product cooler p. Optionally, a post-reactor storage tank or aging storage tank is installed between M and P.

In this embodiment, the urea synthesis effluent from the high pressure section of the urea extraction plant is withdrawn from line 5 and passed through expansion valve D, resulting in decomposition of residual ammonium carbamate and formation of a gas-liquid mixture. The mixture is then sent to heater E where ammonium carbamate is further decomposed. The mixture is sent from heater E to gas-liquid separator F via line 6. The gaseous phase mainly composed of ammonia and carbon dioxide separated off in the separator F is recycled to the carbamate condenser C. There can be more than one heater E and gas-liquid separator F in the urea plant.

The urea product stream is withdrawn from the bottom of separator F via line 8 and further expanded through a second expansion valve G before being fed to an evaporator (not shown) and then into tank H for molten urea. Molten urea can be removed from tank H via line 19, or used for melamine synthesis, by pump I sending it via line 9 and through heater J to urea scrubber K.

The molten urea is fed to the urea scrubber K at a pressure higher than 12.5 MPa but generally lower than about 80 MPa and a temperature above the melting point of urea, preferably lower than about 40 MPa, more preferably lower than about 20 MPa. Although not shown in detail, a cooling jacket may be installed on the scrubber K to ensure additional cooling. The urea scrubber K can also be equipped with an internal cooling device. In the urea scrubber K, liquid urea is brought into contact with the reaction gas from the gas-liquid separator M arranged downstream of the reactor L. The reaction gas consists of carbon dioxide and ammonia, and usually also contains a certain amount of melamine vapour. Molten urea scrubs the off-gas while melamine is returned to reactor L. The tail gas from the scrubber consists mainly of ammonia and carbon dioxide. The tail gas is discharged from the top of the urea scrubber K and returns to the high-pressure section of the urea plant, where urea is prepared by extraction and used as raw material for urea production. Generally, the off-gas stream pressure is practically equal to the pressure in the melamine reactor L, usually higher than about 12.5 MPa. The pressure is generally 0-10 MPa higher than the pressure in the urea reactor, preferably 0-3 MPa higher, more preferably 0-2 MPa higher. The temperature of the gas stream is preferably between about 175°C and 235°C.

The preheated urea is taken out from the urea scrubber K and together with the washed-out melamine, is added to the reactor L through, for example, a high-pressure pump (not shown in detail), and its pressure is higher than 12.5MPa but generally lower than 80MPa, lower than 40MPa It is preferred, more preferably lower than 20MPa. Molten urea can also be conveyed via line 10 to the melamine reactor by gravity by placing a urea scrubber K above reactor L as shown.

The molten urea is treated with heat and pressure in the melamine reactor L to convert urea into melamine, carbon dioxide and ammonia. The temperature is generally in the range of about 300°C to about 500°C, preferably in the range of about 350°C to 425°C. The pressure is above about 12.5 MPa but generally below about 80 MPa, preferably below about 40 MPa, more preferably below about 20 MPa.

Ammonia can be supplied to reactor L, for example, via line 17 . Ammonia fed to the melamine reactor can, for example, be used as a scavenging agent to avoid clogging of the reactor bottom or to avoid formation of melamine condensation products such as melam, melem and cyanuramide or, due to the manner and location of its addition, to facilitate the reaction Mix in vessel L. The amount of ammonia supplied to the reactor is about 0 to about 10 moles per mole of urea, preferably 0-5 moles of ammonia, although in particular about 0.1 to about 2 moles of ammonia per mole of urea may be used. Carbon dioxide and ammonia produced during the reaction and excess ammonia supplied are collected in the separation section. As shown, the separation section may be, for example, a section at the top of the melamine reactor, or it may be, for example, a separator M installed downstream of the reactor. Carbon dioxide and ammonia are separated from the liquid melamine as a gas mixture. This gas mixture is fed to a urea scrubber K to remove entrained melamine vapors and preheat molten urea. Liquid melamine is withdrawn from the melamine reactor L and can be sent, for example, via line 11 to a gas-liquid separator M and then to a product cooler P.

In the gas-liquid separator M, the liquid melamine can then be contacted with about 0.01 to about 10 moles per mole of melamine, preferably about 0.1 to about 2 moles per mole of melamine, via line 18, for example. The residence time in the gas-liquid separator M is generally between 1 minute and 10 hours, but preferably between 1 minute and 3 hours. The pressure in the gas-liquid separator M is generally practically equal to the pressure in the urea to melamine conversion reactor, or may be lower. The temperature may be higher or lower than the reactor temperature, preferably between 200-500°C, especially between 330-440°C. The liquid melamine present in the gas-liquid separator M is discharged from the gas-liquid separator M and sent to the product cooler P via line 14 and expansion valve N. The liquid melamine in the product cooler P is cooled by contact with a cooling medium as described, for example, in U.S. Patent No. 4,565,867 or U.S. Patent No. 5,514,796, the entire disclosures of which are incorporated herein by reference. The cooling medium is preferably ammonia, such as liquid ammonia fed through line 15, for example. The pressure and temperature can also be chosen such that the evaporation of ammonia dissolved in the molten melamine can be used to cool the melamine as described in WO-A-97/20826, the entire disclosure of which is incorporated herein by reference. The melamine is converted to powder in the process and is discharged from the cooling unit via line 16 at the bottom of the product cooler P.

If a post-reactor tank or an aging tank is used, the liquid melamine can again be contacted with about 0.01 to about 10 moles of ammonia per mole of melamine, preferably about 0.1 to about 2 moles of ammonia per mole of melamine. The residence time after the reactor or in the aging vessel is generally between 1 minute and 10 hours, but preferably between 1 minute and 3 hours. The temperature and pressure ranges after the reactor or in the aging vessel are generally the same as the gas/liquid separator. It is best to use lower temperatures.

The evaporation step can be performed by installing an evaporator between the gas-liquid separator and the product cooler. Melamine is converted to gaseous melamine gas during the evaporation step, while by-products, such as melam, remain in the evaporator. Therefore, the amount of by-product impurities in melamine is reduced. As a result, melamine of high purity is obtained. In addition, ammonia may be supplied in the evaporation step, if desired. The gaseous melamine is then cooled by the selected cooling medium, eg ammonia, in the product cooler.

The tail gas of the melamine plant can be fed into the high pressure section of the urea extraction plant. The section receiving the gas mixture may be located, for example, anywhere in the high pressure section from the extractor up to and including the urea reactor itself. Thus, the off-gas stream from the high pressure melamine process can be fed, for example, to a urea reactor, an extractor, a carbamate condenser, an additional pre-extractor between the urea reactor and the extractor, an additional intermediate between the extractor and the carbamate Flash evaporators between salt condensers or in lines between these devices.

Figure 1 illustrates a first embodiment in which the tail gas stream from the high pressure melamine process is fed via line 13 to the carbamate condenser C of a high pressure urea plant. In this embodiment, as with other embodiments of the invention, line 13 may include one or more control valves (not specifically shown). According to this process, no absorption and/or concentration steps are required for the off-gas stream from the melamine plant, since this gas stream is already practically free of water and at a sufficiently high pressure. Example 1 below provides a more detailed explanation of this embodiment of the invention. It will be seen below that, compared with the embodiments of Fig. 2 and 4/6, an advantage of this embodiment and the embodiment of Fig. 3 and 5 discussed later is that the tail gas stream can be directly supplied to the high-pressure section of the urea plant by the melamine plant There is no need for additional, possibly expensive, containers and other equipment to be installed in the installation.

Figure 2 illustrates another embodiment in which the tail gas stream from the high pressure melamine process is fed via line 213 to a flasher Q added between extractor B and carbamate condenser C. This has an advantage if the pressure in the melamine process is much higher than in the urea process.

Figure 3 illustrates another embodiment in which the off-gas stream from the high pressure melamine process is fed via line 313 to the main extractor B of the high pressure urea plant. The advantage is that the tail gas stream is used as extract gas with additional heat recovery. In this exemplary embodiment, the flowsheet of the urea and melamine plant differs from that described above with reference to FIG. 1 .

In another embodiment of the process, as shown in Figures 4 and 6, wherein the tail gas stream from the high pressure melamine process is added to a pre-extractor installed between the urea reactor A and the main extractor B. In this embodiment, the urea synthesis solution is extracted in the pre-extractor R by means of the off-gas stream supplied via line 413 from the high pressure melamine process. The tail gas stream also consists primarily of ammonia and carbon dioxide. This saves additional high-pressure steam and improves extraction results. It has also been found that additional steam is produced in the carbamate condenser C. The pre-extractor R is preferably an adiabatically operated pre-extractor. The latter is an advantage, especially if the melamine plant and the urea plant are highly integrated. A highly integrated plant means that a considerable amount of produced urea is used by the melamine plant, eg more than 50%, more particularly more than 80% of the urea produced by the urea extraction plant is used to produce melamine. It will be appreciated that the "combined" melamine and urea plant of the present invention is not limited to this embodiment.

The tail gas stream feed to the high pressure pre-extractor R of the urea plant is depicted in more detail in FIG. 6 . As in Figure 4, A represents a urea reactor in which urea is produced from ammonia and carbon dioxide. The urea synthesis solution consisting of urea, ammonium carbamate, water and ammonia is supplied to the pre-extractor R via line 2 . An off-gas stream consisting mainly of ammonia and carbon dioxide is sent from the high pressure melamine plant via line 413 and the urea synthesis solution is partially extracted in R. The urea synthesis solution is sent to the main extractor B through the pipeline 2', where the urea synthesis solution is extracted by the extraction medium supplied through the pipeline 20. The urea synthesis solution is separated in this operation into a gaseous stream and a urea solution. The urea solution is discharged via line 5 for further processing. The gas stream mainly composed of ammonia and carbon dioxide from the extractor via line 3 is combined with the gas stream from the pre-extractor R via line 3', also mainly composed of ammonia and carbon dioxide, and together are fed to the carbamate condenser c. The liquid ammonium carbamate solution from the carbamate condenser is sent to urea reactor A through line 4.

Figure 5 illustrates another embodiment in which, as shown, the tail gas stream from the high-pressure melamine process is sent directly to the high-pressure line of the high-pressure urea plant through line 513, for which purpose the line between B and C is preferably a high-pressure line . In this exemplary embodiment, the flow through the equilibrium of urea and melamine is the same as previously described with reference to FIG. 1 .

Production of urea and melamine comprising the feeding of gas consisting mainly of carbon dioxide and ammonia from a high-pressure melamine plant into the high-pressure section of a urea plant, in Netherlands Patent Applications 1003923 and 8 November 1996, respectively, on 30 August 1996 and 8 November 1996 10004475, the complete disclosure of which is hereby incorporated by reference.

EXAMPLES The present invention will be explained in detail with reference to the following examples.

Example 1

At a temperature of 200°C and a pressure of 15 MPa, the N/C ratio is 2.7, and the gas mainly composed of ammonia and carbon dioxide flows out from the top of the melamine scrubber of the high-pressure melamine synthesis process with a production capacity of 5 tons of melamine/hour. This stream is directly added to the carbamate condenser of a 1200t/day urea extraction unit, where the pressure is 14MPa, and the result is compared with the treatment of carbamate from the traditional 0.7MPa low-pressure melamine unit uniting section, The input of 7.6 tons of high-pressure steam (2.7MPa) is less per hour, and the output of urea plant is less than 1.3 tons of low-pressure steam (0.4MPa) per hour. In a conventional combining section, the carbamate stream from the melamine plant is concentrated in order to make it suitable for the urea extraction plant. In addition, 20 tons of high-pressure steam (2.7MPa) can be saved per hour in the combined section of the present invention, but 5.5 tons of high-pressure steam (2.7MPa) need to be increased in the evaporation section of the urea plant. A total of 5.5 tons of high-pressure steam (2.7MPa) can be saved per ton of melamine, and 1.4 tons of low-pressure steam (0.4MPa) can be output less per ton of melamine.

Example 2

At a temperature of 200°C and a pressure of 15 MPa, the N/C ratio is 2.7, and the gas mainly composed of ammonia and carbon dioxide flows out from the top of the melamine scrubber of the high-pressure melamine synthesis process with a production capacity of 5 tons of melamine/hour. This stream is fed directly to the high pressure section of a urea extraction plant. In this example, steam was added to an adiabatically operated pre-extractor installed between the urea reactor and the _CO2 extractor. As a result, compared with Example 1, 2.2 tons of high-pressure steam (2.7MPa) are consumed less in the CO extractor, and 2.4 tons of low-pressure steam (0.4MPa) are less produced in the _carbamate condenser.

Claims (16)

1. A method for preparing urea in a set of urea extraction plants having at least one high-pressure section, wherein said process comprises supplying the gas stream released in the high-pressure preparation of melamine to at least one high-pressure plant of said set of urea extraction plants The step of stage, wherein said gas stream consists essentially of ammonia and carbon dioxide. 2. The method according to claim 1, wherein said high pressure section to which the gas stream is supplied comprises at least one of: a urea reactor, an extractor, a carbamate condenser, a pre-extraction unit installed between the reactor and the extractor evaporator, a flash evaporator installed between the extractor and the carbamate condenser, or a line between any of the above. 3. The process according to claim 2, wherein said high pressure section comprises a high pressure urea reactor and a high pressure extractor, said gas stream being supplied to an adiabatically operated pre-extractor installed between the urea reactor and said extractor . 4. The method according to claim 2, wherein the high pressure section of said urea extraction plant comprises a carbamate condenser, an extractor and a section of pipeline therebetween, wherein said gaseous stream is supplied to said carbamate Condenser, or the pipeline in between. 5. A process according to any one of claims 1-4, wherein said urea extraction unit comprises a urea reactor and said urea reactor is operated at a temperature of 160°C to 220°C. 6. A process according to any one of claims 1-4, wherein said urea extraction unit comprises a urea synthesis reactor operated at a pressure of 12.5-17.5 MPa. 7. A process according to any one of claims 1-4, wherein the temperature of said gas stream released from the melamine process is between 175°C and 235°C. 8. A process according to any one of claims 1-4, wherein the pressure of said gaseous stream released from the melamine process is higher than 12.5 MPa. 9. The method according to claim 8, wherein the pressure of said gas stream released from the melamine process is 0-10 MPa higher than that in said high pressure section. 10. The method according to claim 8, wherein the pressure of said gas stream released from the melamine process is 0-2 MPa higher than that in said high pressure section. 11. A set of urea extraction plant used to prepare urea, the plant has at least one high-pressure section, which includes at least one urea reactor, an extractor, a carbamate condenser, and fluid flow lines connecting them, and the The off-gas stream released in the high-pressure process for the preparation of melamine is supplied to the melamine off-gas line of said high-pressure section, wherein said gas stream mainly consists of ammonia and carbon dioxide. 12. The apparatus according to claim 11, wherein said at least one high-pressure section to which said gas stream is supplied further comprises at least one pre-extractor installed between the reactor and the extractor, and one installed between the extractor and the ammonia flash evaporator between salt condensers, and lines between any of the above equipment. 13. The plant according to claim 11, wherein said high pressure section comprises a pre-extractor installed between the urea reactor and the extractor for adiabatic operation. 14. A device for synthesizing urea and melamine, comprising: A urea plant comprising a high pressure section with a urea reactor, an extractor, a carbamate condenser, and fluid flow lines connecting them; a high-pressure melamine plant comprising a urea scrubber for receiving molten urea, a melamine reactor, and a cooler for producing powdered melamine, and fluid flow lines connecting them; and The off-gas from at least one of said melamine reactor and said urea scrubber is directly supplied to the off-gas supply line of said high pressure section at a pressure equal to or higher than that in said high pressure section. 15. The apparatus according to claim 14, wherein said high pressure section to which said tail gas stream is supplied further comprises at least one pre-extractor installed between the reactor and the extractor, one installed between the extractor and the carbamate Flash evaporators between condensers, and lines between any of the above equipment. 16. The plant according to claim 15, wherein said off-gas supply line is coupled to a pre-extractor installed between the urea reactor and the extractor in adiabatic operation.

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